1. Oak’s Letter: This item is received as a key item after you become the Champion of the Sinnoh League and talk to Professor Oak in Sandgem Town.
2. Azure Flute: This item can be obtained by talking to the woman in the Old Chateau (routes 209 and 210).
3. Eterna Forest: Head to Eterna Forest and use the Azure Flute in front of the flower near the entrance. Shaymin will appear and you can start battling it.
It’s important to note that Shaymin can only be encountered and captured once during normal gameplay in Diamond and Pearl. However, if you have Pokémon Platinum, you can transfer your Shaymin from Diamond or Pearl and then use the Flower Paradise area to change its forme (Land or Sky).